implementing function in 'struct' using C language(not C++)

 In Embedded Programming, such as "AVR" and "PIC", we often can only use C language only(not C++). But knowing how useful and convenient OOP, I googled about how to implement 'member function-like' feature using structure and linking function address. 

 This example is about implementing 1-D Kalman Filter. 

1. First, declare structure including necessary member variables and function. 
 typedef struct _KalmanFilter

{

// Kalman Filter Coefficient Define

float Q;

float R;

// Kalman1 Filter Variables

float x_hat_k;

float x_hat_k_minus;

float x_hat_k_bar;

float P_k_minus;

float P_k_bar;

float K_k;

// Kalman Filter Initial value

float P_k;

int FilterFlag;

// Input

float z_k;

float (*KalmanFiltering)(struct _KalmanFilter* this);

}KalmanFilter;

2. Second, define constructor-like function 

void KalmanFilter_constructor(KalmanFilter* this)

{

this->Q = 0.0001;

this->R = 0.0005;

// Kalman1 Filter Variables

this->x_hat_k = 0;

this->x_hat_k_minus = 0;

this->x_hat_k_bar = 0;

this->P_k_minus = 0;

this->P_k_bar = 0;

this->K_k = 0;

// Kalman Filter Initial value

this->P_k = 0;

this->FilterFlag = 0;

// Input

this->z_k = 0;

this->KalmanFiltering = KalmanFiltering_define;

return;

}

3. Third, define member function declared in the structure

float KalmanFiltering_define(KalmanFilter* this)

{

//Time update equations

this->x_hat_k_bar = this->x_hat_k_minus;

this->P_k_bar = this->P_k_minus + this->Q;

//Measurement undate equations

this->K_k = this->P_k_bar/(this->P_k_bar+this->R);

this->x_hat_k = this->x_hat_k_bar + this->K_k*(this->z_k - this->x_hat_k_bar);

this->P_k = (1-this->K_k)*this->P_k_bar;

//k -> k-1

this->x_hat_k_minus = this->x_hat_k;

this->P_k_minus = this->P_k;

if(!this->FilterFlag)

{//초기화

this->FilterFlag = 1;

return this->z_k;

}

return this->x_hat_k;

}

4. How to use 

// step 1. declare structure-type variables
KalmanFilter AccX_KalmanFilter;

KalmanFilter AccY_KalmanFilter;

KalmanFilter AccZ_KalmanFilter;

// step 2. Initialize structure

KalmanFilter_constructor(&AccX_KalmanFilter);

KalmanFilter_constructor(&AccY_KalmanFilter);

KalmanFilter_constructor(&AccZ_KalmanFilter);

 
while (1)
{
...

        // step 3.  Kalman Filter input setting 

        AccX_KalmanFilter.z_k = (float)g_ACC_X;

        AccY_KalmanFilter.z_k = (float)g_ACC_Y;

        AccZ_KalmanFilter.z_k = (float)g_ACC_Z; 

         // step 4. Kalman Filtering

        g_ACC_F_X = AccX_KalmanFilter.KalmanFiltering(&AccX_KalmanFilter);

        g_ACC_F_Y = AccY_KalmanFilter.KalmanFiltering(&AccY_KalmanFilter);

         g_ACC_F_Z = AccZ_KalmanFilter.KalmanFiltering(&AccZ_KalmanFilter);  
...
}

5. Filtered Result